I don’t know anything about it to be honest. There has to be something that’s preventing the empty fields from being submitted. I did a web search for “php empty fields not submitted” and all I find is stuff on form validation, how to prevent empty fields and this topic. Absolutely nothing on why a field without a value is not being submitted.
Not sure if this has anything to do with it bu what version of PHP are you using?
Do other types of fields do the same thing if you remove the value. For example, if you create a select field that only allows one choice and you allow null. Is this field updated when you remove the value?
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